Nickel-base alloys



Unite 2,794,734 Patented June 4, 1957 ice NICKEL-BASE ALLOYS No Drawing. Application September 8, 1953, Serial No. 379,078

3 Claims. (Cl. 75-170) This invention relates to nickel-base alloys and refers more particularly to improved nickel-silicon alloys.

Nickel-silicon alloys are widely used for the fabrication of equipment for the chemical process industry because of their resistance to corrosion by sulfuric acid which is frequently encountered in the industry. For example, such alloys containing about 8.5% to 10% silicon are resistant to attack by concentrated sulfuric acid at temperatures up to the boiling point. The nickelsilicon alloys have greater ductility and higher resistance to thermal shock than iron-silicon alloys which are also used for resistance to sulfuric acid and for this reason are recommended for use in more severe conditions of service. Both types of material are produced only in the form of castings.

Although the ductility of the nickel-silicon alloys is greater than that of the iron-silicon alloys, improvement in ductility has been sought. Attempts to achieve improved ductility by lowering the silicon content have been successful only at an undesirable sacrifice in corrosion resistance. There is accordingly a continuing demand for improved nickel-silicon alloys having good resistance to corrosion by sulfuric acid and improved resistance to failure by mechanical or thermal shock.

It is the principal object of this invention to satisfy this demand.

The invention by means of which this object is achieved comprises a nickel-silicon alloy containing tantalum in a proportion efiective to impart enhanced ductility and resistance to corrosion. More specifically, the invention comprises alloys containing 4% to 15% silicon; 2% to 25% tantalum; up to 3% aluminum; up to 4% copper; up to 10% iron; the remainder nickel and incidental impurities. A portion of the tantalum, up to about half may be replaced directly by columbium, that is instead of say 10% tantalum, 5% tantalum and 5% columbian may be used. A preferred range of compositions is 5% to 9% silicon; 5% to 15% tantalum or tantalum plus columbium; up to 3% aluminum; up to 4% copper; not more than 10% iron; remainder nickel.

To achieve the desired combination of properties in the alloy of the invention, the silicon and tantalum contents are balanced with respect to each other. Thus, if the silicon content is on the low side of the range, the tantalum content is on the high side of the range. When the silicon content is on the high side of the range, generally less tantalum is needed. Iron is quite detrimental to both mechanical and corrosion-resisting properties of the alloy and desirably is kept well below 10% and whenever possible should be kept below 3%.

Mechanical and corrosion tests of typical alloys embodying the invention demonstrate their improved ductility and their excellent resistance to corrosion. The mechanical tests were performed on dry sand-cast arbitration bars 1.2 inches in diameter and 16 inches long. The bars were not machined, but sand and fins were removed with a hand file and emery paper. The test span was 12 inches, and breaking load, and deflection were measured in pounds and inches respectively. Corrosion tests were conducted on 1 inch diameter disks about inch thick machined from broken arbitration bars. The corrosive media were boiling 77% sulfuric acid and boiling 55% sulfuric acid, and the samples were exposed to these media for three successive 48 hour periods. I

Typical results obtained in such tests are set forth in the following table. In the table the corrosion data are reported in terms of inch penetration per month (ipm) for the third 48 hour period of the tests.

Nominal Composition, Corrosion Rate, Rest Ni and Impurities Breaking Deflec- 1pm Load, tion,

Lbs. Inches Per- Per- Per- Per- 77% 55% cent cent cent cent Acid Acid S1 Ta Ob Fe The data in the table show that in the conventional nickel-silicon alloys (the last three alloys in the table) lowering silicon from 10% to 7% results in lowered corrosion resistance in 77% acid. However, in the alloys of the present invention containing tantalum or tantalum plus columbium substantial increase in toughness and corrosion resistance is obtained in 7% and 8.5% silicon alloys, so much so that in both qualities these lower silicon alloys are far superior to the conventional 10% silicon alloys. The data also show that generally more tantalum is required for good corrosion resistance to 55% acid than for 77% acid.

The alloys of the invention may be made in the induction furnace using pure, preferably electrolytic, nickel, commercially pure silicon and suitable sources of tantalum or tantalum and columbium such as commercially pure metal, or nickel-tantalum alloys, or tantalum-columbium alloys or ferro-columbium or tantalum-ferrocolumbium alloy. Because of the detrimental effect of iron, it is preferred, however, to avoid the use of large amounts of ferroalloys. If desired tantalum may be added as tantalum metal scrap. In making the alloys listed in the table about 0.5% manganese was added for deoxidation.

What is claimed is:

1. A nickel-base alloy consisting of 4% to 10% silicon; 2% to 25% tantalum; columbium from zero to one-half the tantalum content; the sum of tantalum and columbium not exceeding 25 up to 3% aluminum; up to 15 iron; up to 4% copper; the remainder nickel and incidental impurities, the tantalum content of said alloy being on the high side of its said range when the silicon content is on the low side of its said range and being on the low side thereof when the silicon content is on the high side of its said range.

2. A nickel-base alloy consisting of 5% to 9% silicon; 5% to 15 tantalum; columbium from zero to one-half the tantalum content; the sum of tantalum and columbium not exceeding 15%; up to 4% copper; up to 3% aluminum; not more than 10% iron; the remainder nickel and incidental impurities, the tantalum content of said alloy being on the high side of its said range when the silicon content is on the low side of its said range and being on the low side thereof when the silicon content is on the high side of its said range. V

3. A nickel-base alloy consisting'of 5% to 9% silicon; 5% to 15% tantalum; up to 3% aluminum; less than 3% iron; up to 4% copper; the remainder nickel and incidental impurities, the tantalum content of said alloy being on the high side of its said range when the silicon content is on the low side of its said range and being on the low side thereof when the silicon content is on the high side of its said range.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A NICKEL-BASE ALLOY CONSISTING OF 4% TO 10% SILICON; 2% TO 25% TANTALUM; COLUMBIUM FROM ZERO TO ONE-HALF THE TANTALUM CONTENT; THE SUM OF TANTALUM AND COLUMBIUM NOT EXCEEDING 25%; UP TO 3% ALUMINUM; UP TO 15% IRON; UP TO 4% COPPER; THE REMAINDER NICKEL AND INCIDENTAL IMPURITIES, THE TANTALUM CONTENT OF SAID ALLOY BEING ON THE HIGH SIDE OF ITS SAID RANGE WHEN THE SILICON CONTENT IS ON THE LOW SIDE OF ITS SAID RANGE AND BEING ON THE LOW SIDE THEREOF WHEN THE SILCION CONTENT I SON THE HIGH SIDE OF ITS SAID RANGE. 